What we do on a daily basis is definitely boundary negotiation. I like that term for it! We do still have veto power, however (even if we've never used it). I've gotten the sense that Z feels more comfortable knowing that he has the option, and I feel that way a bit myself. It's like having a safety valve; even if the mechanism is unlikely to implode, it's still reassuring to know there's a fail-safe in place.
Now that I'm reflecting on it, part of why that ability is so comforting to Z and I (and not especially important to K) is that it has a bit of our flavor of dominance/submission to it. We do take care of each other quite a bit, and the ability to veto a prospective partner is another way of caring for each other.
"I was thorough when I looked for you, and I feel justified lying in your arms." - Chasing Amy